This invention relates to a novel novolak resin containing silicon and a resist material which comprises the novolak resin and is useful in forming fine patterns in the manufacture of microelectronic devices, and to a pattern forming method in which the novel resist material is used to form a resist pattern that servies as a mask for dry etching of an underlying organic layer.
In the manufacture of microelectronic devices such as semiconductor integrated circuit devices and bubble memory devices, optical lithography and electron-beam lithography are widely used to from fine patterns. In such techniques the thickness of a resist layer is an important factor in realization of high resolution patterns. It is well known that the use of a thick resist layer results in low resolution mainly by reason of reflection from the substrate in the case of optical lithography and scattering of electrons in the case of electron-beam lithography. However, when the resist layer is made thin to thereby obtain a high resolution pattern there arises a problem that in the subsequent etching process the resistivity of the resist pattern is insufficient. This is particularly serious when a dry etching technique such as reactive ion etching or ion beam etching is employed in the subsequent process since conventional resist materials are generally low in resistivity to dry etching.
Besides, it is not seldom that the surface of the substrate has steps, and in such cases it is difficult to form fine patterns by a conventional lithography method even if the overlying resist layer is made fairly thin.
To solve the above described problems, J. M. Moran et al have proposed a three-layer technique in J. Vacuum Science and Technology, 16, No. 6, 1620 (1979). Accordingly to this three-layer technique the first layer which covers the substrate surface and provides a flat surface is a sufficiently thick layer of an organic material, and the intermediate layer is formed of an inorganic material that can not easily be etched by dry etching with oxygen, such as silicon, silicon dioxide or silicon nitride. The third or top layer is a thin resist layer. In the patterning process, first the resist layer is exposed to light, X-ray or electron-beams and developed to produce a resist pattern. Next, the intermediate layer is subjected to etching with the resist pattern as a mask. Finally the thick organic layer is etched by reactive sputter etching using oxygen with the patterned intermediate layer as a mask. By this method a high resolution pattern initially delineated in the thin resist layer can be transferred to the organic layer. However, it is a disadvantage of the three-layer method that the processing operations become complicated and time-consuming mainly because of the addition of the intermediate layer which is formed by vacuum deposition, sputtering or plasma CVD method.
If it is practicable to use a resist material which is sufficiently resistive to dry etching the above described three-layer structure can be simplified to a two-layer structure. However, conventional resist materials do not meet this desire. Polydimethylsiloxane is known as resistive to dry etching, but this polymer is unsuitable for practical use as a resist material because it is liquid at room temperature.
As a practical solution of the above problems, J. Electrochem. Soc., 130, 1962 (1983) and European patent application publication No. 0096596 have disclosed a pattern forming method of a two-layer type using a new resist material which comprises a polymer having trialkylsilyl groups or dimethylphenylsilyl groups. This resist material is highly resistive to reactive oxygen ion etching and is sensitive to electron beams and deep UV rays, and the simple two-layer structure consisting of a relatively thick organic layer laid on a substrate and a thin resist layer is favorable for industrial practice. Howevr, this pattern forming method is practicable only when the resist pattern is a positive pattern.